By applying these formulas recursively, Sk can be expressed solely in terms of the Ek, which is often desirable. For example, since S1=E1, S2=E12-2⁢E2, and then S3=(E12-2⁢E2)⁢E1-E1⁢E2+3⁢E3=E13-3⁢E1⁢E2+3⁢E3, and so on.